After a cold internal combustion engine starts, a portion of the metered fuel condenses on areas in the induction pipe which are still cold and on the cylinders of the internal combustion engine. A further portion leaves the cylinders uncombusted with the exhaust gas as a consequence of inadequate vaporization before the ignition. The composition of the mixture which is combusted in this phase can become much leaner owing to these effects and this can lead to problems in the operating performance of the engine.
In order to avoid such problems, which include, for example, an unsatisfactory power output, juddering during acceleration or stalling during idling, the operating mixture in this phase is usually enriched with fuel as a function of time or temperature. Such a procedure is disclosed, for example, in U.S. Pat. No. 4,205,635. Here, a comparatively excessive enrichment is less critical for disturbance-free operation of the engine than enrichment which is too weak in comparison. In the time between the start of the internal combustion engine and the activation of a lambda control, which is usually less than a minute, the degree of optimum enrichment is also dependent on the properties of the fuel being used. These fuel properties can fluctuate regionally and in dependence on the time of year. A strictly controlled enrichment is customary to cover the entire range of fuel qualities which vary regionally and in dependence on the time of year. However, enrichment which exceeds the particular extent of enrichment required leads to increased emissions of toxic substances.